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Physics Procedia

Advanced Four-Layered Josephson Junctions for Digital Applications

V. Lacquaniti, Natascia De Leo, Matteo Fretto, A. Sosso, I. V. Boĭlo, Mikhail A. Belogolovskii

Published Jan 1, 2012• 4 citations
Publisher

Abstract

Device-modeling physics of four-layered Nb/Al-ultra-thin AlO x barrier-Nb Josephson structures is discussed, empha sizing practical applications. The temperature effect on the minigap induced in a proximized Al layer and that on a critical voltage across the SNIS junction is numerically simulated. The McCumber-Stewart parameter is found to decreasefromabout2at1.7Ktovalues nearunityat4.2K, becauseofthe temperature suppressedsubgap resistance.Due to the single-valued current-voltage dependence and comparatively high critical voltage values, the overdamped SNIS junctionshave demonstrated well-developed rf-induced featuresinthe current-voltage characteristicsevenfarabove4.3 K, appealing properties for programmable Josephson voltage standard applications.

DOI
10.1016/j.phpro.2012.06.053

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IEEE Transactions on Applied Superconductivity•Jun 1, 2009•27 citations
Device-modeling physics of four-layered Nb/Al-ultra-thin AlO x barrier-Nb Josephson structures is discussed, empha sizing practical applications. The temperature effect on the minigap induced in a proximized Al layer and that on a critical voltage across the SNIS junction is numerically simulated. The McCumber-Stewart parameter is found to decreasefromabout2at1.7Ktovalues nearunityat4.2K, becauseofthe temperature suppressedsubgap resistance.Due to the single-valued current-voltage dependence and comparatively high critical voltage values, the overdamped SNIS junctionshave demonstrated well-developed rf-induced featuresinthe current-voltage characteristicsevenfarabove4.3 K, appealing properties for programmable Josephson voltage standard applications.
SNS Nb/Al-AlOx/Nb Josephson junctions have been recently developed with Al layer thicknesses 30- 100 nm , for which non-hysteretic I-V characteristics have been obtained by oxidation ranging from 150 to 200 Pa.s. The Stewart-McCumber parameter of the junctions can be lower or higher than unity, depending on the above-mentioned conditions. Typical Jc values of several tens of kA/cm2 and RnIc values up to 0.5-0.7 mV at 4.2 K have been obtained, suitable for digital applications. Two interesting features of this technology are: i) the absence of external shunt resistance that can allow higher integration and; ii) the reproducibility of technological parameters that is connected to a least dependence of the Josephson critical current on the barrier thickness. In this work, we will present a detailed analysis of speed and jitter that can be expected from basic digital circuits based on such a technology. This analysis is based on the accurate experimental extraction of typical quantities, like the Stewart-McCumber parameter or specific capacitance. In particular, the influence of the temperature of operation on the performance of elementary cells will be assessed. A comparisonwith similar cells based on externally-shunted Josephson junctions will be presented in order to evaluate the ranges of parameters for which this technology exhibits some clear advantages. In particular we will show the technological constraints that need to be met in order to build circuits that can compete with existing technologies
Improved Characteristics of Intrinsically Shunted Nb/Al−AlOx−Nb{\rm Nb/Al}-{\rm AlO}_{x}-{\rm Nb}Nb/Al−AlOx​−Nb Josephson Junctions
Josephson junctions with non-hysteretic current-voltage characteristics form the basis of most superconducting electronic circuits including RSFQ logic and programmable Josephson voltage standards. In contrast to conventional SIS devices, Nb/Al-AlOx-Nb (SNIS) junctions with an additional comparatively thick Al interlayer and a comparatively thin barrier AlOxare intrinsically shunted at T ≥ 4.2 K . In this contribution, we provide experimental and theoretical arguments proving that this finding is mainly explained by a broad distribution of highly-conductive barrier transparencies with a significant effect from nearly ballistic modes. An additional advantage of the proposed SNIS junctions is possibility to tune the critical voltage value by modifying Nb and/or Al film thicknesses. With observations of wide Shapiro steps up to 1.25 V at 6.3 K we show that this type of Josephson junctions can be successfully used at temperatures above 4.2 K. The presence of well-developed quantized voltage features even at 7.2 K means that Nb/Al-AlOx-Nb devices can successfully operate far above the liquid helium temperature and, in principle, are compatible with two-stage cryocoolers.
The overdamped Nb/Al-AlOx/Nb superconductor-normal metal-insulator-superconductor SNIS Josephson junctions recently developed at INRIM exhibit high values of critical current density and characteristic voltage, making these devices appealing both for metrology and digital electronics. High current densities could allow reduction of junction size, increasing the integration level. In this work authors present first results concerning the realization of sub-μm SNIS including a nanolithographic technique (Electron Beam Lithography, EBL) into the conventional process, to reduce junction dimensions. Fabrication and electrical parameters have been evaluated considering the scaling of the areas of junctions, furthermore RF and temperature behaviour have been studied.
An interesting feature of Superconductor Normal metal Superconductor - Josephson junctions for digital applications is due to their non-hysteretic current-voltage characteristics in a broad temperature range below Tc. This allows to design Single-Flux-Quantum (SFQ) cells without the need of external shunts. Two advantages can be drawn from this property: first the SFQ cells can be more compact which leads to a more integrated solution towards nano-devices and more complex circuits; second the absence of electrical parasitic elements associated with the wiring of resistors external to the Josephson junctions increases the performance of SFQ circuits, in particular regarding the ultimate speed of operation. For this purpose Superconductor Normal metal Insulator-Superconductor Nb/Al-AlOx/Nb Josephson junctions have been recently developed at INRiM with aluminum layer thicknesses between 30 and 100 nm. They exhibit non-hysteretic current-voltage characteristics with RnIc values higher than 0.5 mV in a broad temperature range and optimal Stewart McCumber parameters at 4.2 K for RSFQ applications. The main features of obtained SNIS junctions regarding digital applications are presented.
Motivated by a search for a suitable technology to fabricate Josephson junctions with a tunable damping regime, we performed a systematic study of the temperature effect on the critical current in Nb/Al-AlO x -Nb heterostructures with a nanometer-thick Al interlayer. For Al layer thicknesses ranging from 40 to 110 nm, we have observed a transition from hysteretic (below 4.2 K) to non-hysteretic (above 4.2 K) current-voltage curves. Measured supercurrent-vs-temperature characteristics which significantly differ from those of traditional SIS and SNS devices are interpreted in terms of the superconducting proximity effect between Al and Nb films. Thermal stability and good reproducibility of our junctions are demonstrated.
Niobium-based junctions are still the best candidates for many small-scale applications of superconductivity and in particular, for the design and the fabrication of programmable voltage standards and D/A converters of fundamental accuracy. Non-hysteretic junctions with a large current density JC, and a reasonable characteristic voltage Vc are required, while, in order to make these circuits available to a wider market, operation at temperatures above 4.2 K would be important. In this work we report our results achieved on Nb/Al-AlOX/Nb junctions whose properties are studied in order to optimize the aforementioned features. Their behavior is analyzed showing how the overdamped characteristic is obtained with a less critical structure compared to other type of junctions. Moreover, using the scattering matrix approach, we have calculated a deviation of superconducting current-phase relation in such a SNIS Josephson junction from sinusoidal one and its effect on the current amplitudes of Shapiro steps as function of temperature. Measurement of the step dependence on temperature are also reported, where we considered the possibility of using bothn= 1 andn= 2 RF step above 4.2 K, for a possible reduction of the number of junctions in array circuits.